1. Field of the Invention
The present invention relates to a common-rail, fuel-injection system in which fuel under high-pressure in a common rail is injected into the combustion chambers of engines.
2. Description of the Prior Art
Among various types of fuel-injection systems for engines is conventionally well-known a common-rail, fuel-injection system in which the fuel stored under high-pressure in the common rail is applied to the injectors, which are in turn actuated by making use of a part of the high-pressure fuel as a working fluid to thereby spray the fuel applied from the common rail into the combustion chambers out of discharge orifices formed at the tips of the injectors.
An example of a conventional common-rail, fuel-injection system will be explained below with reference to FIG. 9. A fuel feed pump 6 draws fuel from a fuel tank 4 through a fuel filter 5 and forces it under a preselected intake pressure to a high-pressure, fuel-supply pump 8 through a fuel line 7. The high-pressure, fuel-supply pump 8 is of, for example, a fuel-supply plunger pump driven by the engine, which intensifies the fuel to a high pressure determined depending on the engine operating conditions, and supplies the pressurized fuel into the common rail 2 through another fuel line 9. The fuel, thus supplied, is stored in the common rail 2 at the preselected high pressure and forced to the injectors 1 through injection lines 3 from the common rail 2. The engine illustrated is a six-cylinder engine. There are six injectors 1, each to each cylinder, to spray the fuel into the combustion chambers formed in the cylinders. The engine is not limited to the six-cylinder type, but may be the four-cylinder engine.
The fuel relieved from the high-pressure, fuel-supply pump 8 is allowed to flow back the fuel tank 4 through a fuel-return line 10. The unconsumed fuel remaining in each injector 1 out of the fuel fed through the fuel line 9 into the injectors 1 may return to the fuel tank 4 through a fuel-recovery line 11. The controller unit 12 is applied with various signals of sensors monitoring the engine operating conditions, such as a crankshaft position sensor for detecting the engine rpm Ne, an accelerator pedal sensor for detecting the depression Ac of an accelerator pedal, a high-pressure fuel temperature sensor and the like. In addition, the sensors for monitoring the engine operating conditions include an engine coolant temperature sensor, an intake manifold pressure sensor and the like. The controller unit 12 is also applied with a detected signal as to a fuel pressure in a common-rail 2, which is reported from a pressure sensor 13 installed in the common rail 2.
The controller unit 12 may regulate the fuel injection characteristics on the injectors 1, including the injection timing and the quantity of fuel injected, depending on the applied signals, so as to operate the engine with the optimal injection timing and quantity of fuel injected per cycle in conformity with the recent engine operating conditions, thereby allowing the engine to operate as fuel-efficient as possible. As the injection pressure of the fuel sprayed out of the injectors is substantially equal with the common rail pressure, the injection pressure defining, in combination with the injection duration, the quantity of fuel injected per cycle may be controlled by operating a fuel flow-rate control valve 14, which is to regulate the quantity of high-pressure fuel supplied to the common rail 2. In case the injection of fuel out of the injectors 1 consumes the fuel in the common rail 2 or it is required to alter the quantity of fuel injected, the controller unit 12 actuates the fuel flow-rate control valve 14, which in turn regulates the quantity of delivery of the fuel from the high-pressure, fuel-supply pump 8 to the common rail 2 whereby the common rail pressure recovers the preselected fuel pressure. Regulating a duration during which the fuel flow-rate control valve 14 is open results in controlling the quantity of the fuel fed into the common rail 2 through the fuel line 9 out of the fuel discharged from the high-pressure, fuel-supply pump 8.
Referring to FIG. 10, the injector 1 is comprised of an injector body 21, and an injection nozzle 22 mounted to the injector body 21 and formed therein with an axial bore 23 in which a needle vale 24 is fitted for a sliding movement. The high-pressure fuel applied to the individual injector 1 from the common rail 2 through the associated injection line 3 is allowed to flow into fuel passages 31, 32 formed in the injector body 21 and communicated with the associated injection line 3 through a high-pressure fuel inlet coupling 30. The high-pressure fuel further reaches the discharge orifices 25, formed at the tip of the injection nozzle 22, past a fuel sac 33 formed in the injection nozzle 22 and a clearance around the needle valve 24 fitted in the axial bore 23. Therefore, the instant the needle valve 24 is lifted to open the discharge orifices 25, the fuel is injected out of the discharge orifices 25 into the combustion chamber while the unconsumed fuel remaining in the injector 1 may return to the common rail 2 through a fuel-recovery line 11.
The injector 1 is provided with a needle-valve lift mechanism of pressure-control chamber type in order to adjust the lift of the needle valve 24. The high-pressure fuel fed from the common rail 2 is partly admitted into a pressure-control chamber 40. The injector 1 has at the head section thereof a solenoid-operated valve 15, which constitutes an electronically-operated actuator to control the inflow/outflow of the high-pressure fuel with respect to the pressure-control chamber 40. The controller unit 12 makes the solenoid-operated valve 15 energize in compliance with the engine operating conditions, thereby adjusting the fuel pressure in the pressure-control chamber 40 to either the high pressure of the admitted high-pressure fuel or a low pressure released partially in the pressure-control chamber 40. On energizing a solenoid 38 in the solenoid-operated valve 15 by an exciting signal, for example, a current value, which is a control signal applied from the controller unit 12 via a signaling line 37, the armature 39 rises to open a valve 42 arranged at one end of a fuel-leakage path 41. The fuel fed in the pressure-control chamber 40 is allowed to discharge past the opened valve 42 to thereby release partially the high fuel pressure.
A control piston 44 is arranged for axial linear movement in an axial recess 43 formed in the injector body 21 of the injector 1. At the event the pressure-control chamber 40 is under the high pressure, the fuel pressure forces the needle valve 24 downward to close the discharge orifices 25. When the solenoid-operated valve 15 is energized to cause the fuel pressure inside the pressure-control chamber 40 to reduce, the resultant force of the fuel pressure in the pressure-control chamber 40 with the spring force of the return spring 45, acting on the control piston 44 so as to pushing it downward, is made less than the fuel pressure acting on both a tapered surface exposed to a fuel sac 33 and the distal end of the needle valve 24, whereby the control valve 44 moves upwards. As a result, the needle valve 24 lifts to allow the fuel to spray out of the discharge orifices 25. The quantity of fuel injected per cycle is defined dependent on the fuel pressure in the fuel passages and both the amount and duration of lift of the needle valve 24.
The actual quantity of fuel injected is calculated based on an amount of pressure drop occurring nearby the fuel injection. The controller unit 12 adjusts a duration during which the fuel-injection nozzle is held open, so as to make the found actual quantity of fuel injected a desired quantity of fuel to be injected conforming with the engine operating conditions. Calculating the actual quantity of fuel injected is disclosed in Japanese Patent Laid-Open No. 186034/1987. According to a fuel-injection control described in the above citation, the pressure-control chamber is of a control-rod pressure chamber of a volumetric component, so that pressurizing or depressurizing owing to variations in volume causes the needle valve to move upward and downward thereby injecting the fuel out of the discharge orifices at the end of the fuel-injection valve. As an alternative, the actual quantity of fuel injected may be calculated in accordance with the amount of pressure drop in the common rail, the common rail pressure just before the pressure fall and the fuel temperature. Control of the open and the closure of the fuel-injection valve is carried out by multiplying the desired quantity of the injected fuel by the ratio of the desired quantity of the injected fuel to the actual quantity of fuel injected, thereby finding a corrected, desired quantity of fuel injected.
On the other hand, disclosed in co-pending senior patent application in Japan, or Japanese Patent Laid-Open No. 77924/1998 is a fuel-injection apparatus in which a valve having a tapered valve head in a pressure-control chamber makes the supply and relief of the high pressure in the pressure-control chamber. The fuel-injection apparatus cited above includes a valve stem extending into a pressure-control chamber past a discharge passage for relieving the fuel pressure from the pressure-control chamber. The valve stem is provided at its end with the tapered valve head having a valve face, which moves away from and reseats against a valve seat at the ingress end of the discharge passage to thereby control the fuel pressure in the pressure-control chamber as well as the relief of the fuel pressure. The instant the valve is made open to allow the high-pressure fuel to leak out the pressure-control chamber, the needle valve starts to lift thereby injecting the fuel out of the discharge orifices at the distal end of the injector. In our senior concept as cited briefly above, as the actuator has no pressure affecting the fuel in the pressure-control chamber, there is no need of paying attention to the sealing performance in the pressure-control chamber. This is advantageous to dealing with the requirements as to the high fuel-injection pressure used in the modern diesel engines.
Nevertheless, in the common-rail, fuel-injection system as described above, the actual quantity of fuel injected out of the discharge orifices at the distal end of the injector is a part of the fuel fed to the injector, whereas another part of the fuel in the injector becomes a dynamic leakage flowing out from the pressure-control chamber to the relatively low-pressure side past the valve. It is, therefore, substantially impossible to find the actual quantity of fuel injected per cycle, based on only the pressure fall in the common rail pressure. This makes it very hard to control the operation of the injector so as to provide the desired quantity of fuel injected, causing a major problem of failure to achieve the optimal fuel-injection control, which results in making the exhaust gases performance and noise control worse.
In the common-rail, fuel-injection system, accordingly, in order to calculate by the controller unit the desired quantity of fuel to be injected in compliance with the detected engine operating conditions, it is necessary to find how part of the fuel entering the injector from the common rail is the actual quantity of fuel injected really out of the injector, and control the fuel injection so as to make the desired quantity of fuel injected of the actual quantity of fuel injected.